The application development for bringing fuel cells into practical use has been promoted in recent years. The application of fuel cells include: power generation systems such as power sources for home use and distributed power generation; cogeneration systems in which the power generation system is combined with a technology to utilize waste heat; power sources for driving mobile units such as automobiles; and power sources for mobile terminal units such as electronic devices.
Among the fuel cells under development for these applications, the most promising is a solid polymer fuel cell that utilizes a solid polymer electrolyte membrane as an electrolyte membrane and is capable of operating in a temperature range from room temperature to about 80° C.
In order to bring these fuel cells into practical use, it is necessary to further reduce the size of fuel cell systems. To this end, it is effective to operate fuel cells without humidification in a temperature range from room temperature to 200° C. This is because if a fuel cell can be operated without humidification, a humidifier can be omitted in the fuel cell system. Moreover, if the operation of a fuel cell can be started from room temperature, the load applied to the heater at the start-up of the fuel cell can be reduced as compared to the fuel cell system in which the operation of a fuel cell cannot be started from room temperature. As a result, the size of the heater can be reduced or the heater itself can be omitted. Furthermore, if the operable temperature range, which is currently up to about 80° C., can be further increased, the power output generated by the fuel cell can be increased, and consequently the size of power generation unit can be reduced.
As just described, if a fuel cell capable of operating without humidification in a temperature range from room temperature to 200° C. can be achieved, further size reduction can be achieved in fuel cell systems.
The proton conducting material, which is a component of a fuel cell, in conventional techniques will be described.
Currently, the most widely used electrolyte membrane for solid polymer fuel cells is a fluorine polymer containing a strongly acidic functional group in its side chain (i.e., perfluorosulfonic acid polymer) such as NAFION (trademark) available from E.I. Du Pont de Nemours & Co. Inc. of the United States. This perfluorosulfonic acid polymer when humidified exhibits a very high proton conductivity (electrical conductive rate or proton conductive rate) of about 10−1 S/cm in a temperature range from room temperature to not greater than 100° C.
However, the perfluorosulfonic acid polymer requires humidification. And at a temperature of not less than 100° C., its conductivity decreases significantly. Therefore, the perfluorosulfonic acid polymer has a problem that it cannot be used in such a condition. This is because the perfluorosulfonic acid polymer absorbs water when humidified, whereby the water forms a path for conducting ions, but at a temperature of not less than 100° C., the water evaporates and thus the path for conducting ions disappears.
A description is now given of a specific approach for developing a proton conducting material capable of functioning in a stable manner even without humidification in a wide temperature range from room temperature to about 200° C.
In order to achieve operation without humidification, it is necessary to use a proton conducting medium except water. A candidate of the proton conducting medium is an organic compound having proton conductivity such as a heterocyclic organic compound. Specifically, imidazole, pyrazole and their derivatives are well known.
These organic compounds, however, have the following problem. Imidazole, for example, which has a melting point of 90° C., melts and turns into a liquid at a temperature equal to or greater than its melting point, and exhibits a high proton conductivity of 10−3 S/cm or greater. At a temperature lower than its melting point, however, it turns into a solid crystal, and its proton conductivity decreases by one to two digits or more (see Non-Patent Document 1, for example). In short, imidazole does function even without humidification, but it has a problem that there is a limitation in its operable temperature and its function decreases significantly at a temperature from room temperature to 90° C.
As just described, a heterocyclic organic compound such as imidazole can function as a proton conducting material without humidification, but it has the problem that its function decreases significantly at a temperature equal to or less than its melting point. This problem applies also to a proton conducting material that has a melting point and turns into a solid crystal at a temperature lower than its melting point.
In order to solve this problem, for example, the inclusion of a heterocyclic organic compound in an acidic polymer membrane is proposed (see, for example, Patent Documents 1 and 2). Specifically, there is proposed a proton conducting material obtained by including a heterocyclic organic compound, namely imidazole, in a membrane made of a polymer having an acidic group, such as polyvinyl phosphate or sulfonated polyether ketone. Also, a method for producing a proton conducting material is proposed in which a proton conductive compound (e.g., cesium hydrogen sulfate) is combined with porous silica to produce a proton conducting material (see, for example, Patent Documents 3 and 4).
Electrodes for a fuel cell will be described below.
An electrode for a fuel cell is composed mainly of three components: a catalyst, a proton conducting material and an electron conductor. A typical example is an electrode in which catalyst-carrying carbon particles and a perfluorosulfonic acid polymer serving as a proton conducting material are mixed. For designing such electrode, it is considered important to increase the contact area between the catalyst particles and the proton conductive polymer and to take into account the formation of gas channels for supplying reaction gases (see, for example, Non-Patent Document 2 and Patent Document 5).
Non-Patent Document 2 and Patent Document 5 disclose that it is important to increase the contact area between the catalyst particles and the proton conductive polymer by preventing the aggregation of the catalyst particles by controlling the ratio of the amount of the catalyst particles and that of the proton conductive polymer or the porous structure of an electrode. More specifically, the above documents disclose it is necessary to introduce a proton conductive polymer into 0.04 to 1.0 μm diameter pores of an electrode in order to prevent the aggregation of the catalyst particles and to cover the surface of the catalyst particles with the proton conductive polymer.
The electrodes disclosed in Non-Patent Document 2 and Patent Document 5 will now be described with reference to the accompanying drawing. FIG. 6 is a schematic enlarged diagram of the microstructure of a conventional electrode. In FIG. 6, catalyst particles 51 are carried on carbon particles serving as an electron conductor. A plurality of the carbon particles 52 carrying the catalyst particles 51 (hereinafter referred to as “catalyst-carrying carbon”) are aggregated, forming a catalyst-carrying carbon aggregate. Proton conductive polymers 53 are arranged such that they cover the catalyst-carrying carbon aggregate. The proton conductive polymers 53 are in contact with any one of the plurality of catalyst particles 51 and the carbon particles 52.
In order to prevent the aggregation of the catalyst particles 51 to increase the contact area between the catalyst particles 51 and the proton conductive polymers 53, all the catalyst particles 51 and all the carbon particles 52 have to be covered with the proton conductive polymers 53. As clearly seen in FIG. 6, the catalyst particles 51 that are contained in the catalyst-carrying carbon aggregate are not covered with the proton conductive polymers 53. Accordingly, the catalyst particles 51 inside the aggregate are not involved in the reaction. As discussed above, the important point is to prevent the aggregation of the catalyst particles and to increase the contact area between the catalyst particles 51 and the proton conductive polymers 53, and Patent Document 5 proposes a method for designing such electrode.
Patent Document 5 points out that the structure of the electrode as described above needs to be designed differently according to the type of electrolyte (e.g., phosphoric acid or polymer electrolyte).
Patent Document 6 proposes a porous material for an ion conductor.    Non-Patent Document 1: The Journal of Chemical Physics Volume 52, Number 6, 3121-3125    Non-Patent Document 2: Journal of ElectroChemical Society Volume 142, Number 2, 463    Patent Document 1: U.S. Pat. No. 6,264,857    Patent Document 2: Japanese Laid-Open Patent Publication No. 2004-185891    Patent Document 3: Japanese Laid-Open Patent Publication No. 2004-247253    Patent Document 4: Japanese Laid-Open Patent Publication No. 2004-2114    Patent Document 5: Japanese Laid-Open Patent Publication No. Hei 9-92293    Patent Document 6: Japanese Laid-Open Patent Publication No. 2004-259593